Atom Smashers

What would you do if you could fire a bullet at a speed of nearly 186,000 miles per second? Shoot it into something just to see what happens? That’s pretty close to what some UCLA scientists are doing. More accurately, they take two beams of protons, each traveling so fast that they could circle the globe more than seven times in a single second, then smash them together in the center of a huge detector and take a picture of the debris. As if that weren’t challenging enough, they do it 40 million times a second.

So why would anyone do this? What scientific purpose does it serve? Well, for one, the temperature at the center of these collisions is the hottest ever achieved by mankind. It is 100,000 times hotter than the center of the sun. It is 10 times hotter than the center of a supernova, which is the explosion of a star with a flash bright enough to be seen halfway across the visible universe. A temperature like this was last common a scant trillionth of a second after the Big Bang. In a real sense, scientists are re-creating the conditions of the very birth of the universe and studying it in incredible detail. These collisions also enable scientists to peer deep inside matter, looking at things that are one-hundred-millionth the size of an atom, in an effort to understand the most fundamental building blocks of matter. By any measure, these studies probe the frontier of human knowledge.

The facility that enables scientists to do this is called the Large Hadron Collider, or LHC. It is located just outside Geneva, Switzerland, and is the largest particle accelerator ever built — a huge ring about 17 miles in circumference. Collisions occur inside four large detectors that are essentially large, fast cameras, frantically taking photos of the collisions. The particle detector on which UCLA scientists work is called the Compact Muon Solenoid, or CMS. This 100-megapixel camera stands 50 feet high, is 70 feet long, and weighs about 14,000 tons. It’s as big as a medium-size building.

Finding The Elusive Higgs Boson

Three UCLA physics professors, Robert Cousins, Jay Hauser and David Saltzberg, are in the business of asking and answering hard questions. This trio of scientific sleuths leads a group of UCLA researchers as part of an international collaboration to conduct research using the CMS detector. In 2012, that collaboration resulted in the discovery of the Higgs boson, an elusive subatomic particle that was the last missing piece of the wildly successful Standard Model of particle physics. The Higgs boson is the particle that gives mass to all subatomic particles, while the Standard Model is our best theory of the rules that govern the matter and energy of the universe. The discovery of the Higgs boson led to the 2013 Nobel Prize in physics for the team that predicted the particle’s existence back in 1964.

Hauser says, “When the LHC started colliding protons in 2010, we all knew that finding the Higgs particle would be very difficult. Performing better than we’d hoped, the accelerator delivered a lot of collisions, and the detector and the ingenuity of our data analysts exceeded our expectations. It really was a tour de force of experimental science.”

The red circle shows the scale of the LHC. The area under which the tunnel for
CERN’s LHC can be found is shown, near Geneva and Lac Léman. Seen in the background are the French Alps, with Mont Blanc. Photo courtesy of CERN.

The LHC ran from 2010 to February 2013, and CMS-affiliated scientists accumulated data that have led to more than 400 publications, a fraction of which were related to the discovery of the Higgs boson. The others investigated the behavior of other subatomic phenomena, which resulted in doctoral degrees for four UCLA graduate students. Saltzberg says, “The UCLA Ph.D. students on CMS were in the experimental trenches at CERN. They built apparatuses that recorded signals from the beam collisions. And they analyzed data, looking for rare anomalies that could have indicated a big discovery.”

While data already recorded and analyzed were an unqualified success, for the past two years the LHC has been shut down for refurbishments, retrofits and upgrades. The UCLA CMS group and their collaborators have used this time to work on the CMS detector to upgrade its capabilities.

While Cousins, Hauser and Saltzberg currently helm the group, they are assisted by about a dozen researchers, postdoctoral associates, and graduate and undergraduate students. The UCLA researchers built muon detectors, which played a central role in the discovery of the Higgs boson.